Dry kilns are used for controlled drying of green lumber, i.e., lumber that is newly cut from logs. Uncontrolled drying can result in the lumber warping and splitting. Thus, to maintain grade, the lumber is dried through a process of controlled heating. The heating process involves control over the temperature, humidity and circulation of the air in the kiln. Temperature and humidity in particular have to be monitored and changed throughout the process as the lumber transcends from a wet condition to a dry condition. Even further, the process of drying will be different for different types of lumber.
The controls and procedures for drying lumber in a dry kiln have become very sophisticated but heretofore the process has always relied on venting to the atmosphere. The air in the dry kiln becomes saturated with moisture. The outgoing moist air is vented to the atmosphere and replaced with drier air from the atmosphere which is drawn into the kiln and heated to the desired temperature.